1. Field of the Invention
The present invention relates to an optical scanning apparatus provided in an image forming apparatus such as a copying machine or a printer, and an image forming apparatus comprising the optical scanning apparatus.
2. Description of the Related Art
An optical scanning apparatus comprised in an electrophotographic image forming apparatus drives a semiconductor laser in accordance with input image data and forms an electrostatic latent image corresponding to the input image data on a photosensitive member. To achieve operation at higher speed and higher resolution, it is necessary to rotate a polygon mirror at higher speed, which is used to scan a laser beam from the semiconductor laser on the photosensitive member, or to increase the number of emission points in the semiconductor laser, from which laser beams are emitted, thereby increasing the number of laser beams which simultaneously scan on the photosensitive member.
There is, however, a limit to higher speed rotation of the polygon mirror in consideration of generated heat, generated noise and strength of the polygon mirror itself.
Recently, therefore, an arrangement using a surface emitting laser array (VCSEL: Vertical Cavity-Surface Emitting Laser) as a light source capable of generating a large number of laser beams, in place of conventional laser diodes in end face emitting type has been commercialized.
Since the laser beam output characteristics of a semiconductor laser are sensitive to a change in ambient temperature, the laser beam output greatly varies due to the change in ambient temperature or the self-heating even if the semiconductor laser is driven at a constant current. To cope with the temperature dependence of the laser beam output, a photodiode called a monitor diode is incorporated with the laser diode in end face emitting type in the semiconductor laser package. The monitor diode monitors an output of back light of the semiconductor laser to control the drive current, thereby being able to keep the laser beam output constant. A driving circuit which performs laser beam Amount control of the semiconductor laser having such a feedback mechanism is called an APC (Auto Power Control) circuit.
The VCSEL can emit laser beam only in one direction owing to structural limitations, and hence cannot perform APC by monitoring the output of back light unlike the laser diode in end face emitting type.
Japanese Patent Laid-Open No. 2003-215485 proposes the following light amount detection arrangement. As shown in FIG. 12, there is proposed an arrangement in which a part of a laser beam emitted from a light source 151 and traveling to a photosensitive member 154 is split using a half mirror 153 placed in an optical path from the light source 151 to a polygon mirror 152, the split laser beam is guided to a photodiode (to be referred to as a “PD” hereinafter) 155, and a light amount of the laser beam is detected by the photodiode 155.
Japanese Patent Laid-Open No. 9-230259 proposes an arrangement in which a light amount control is performed using a BD sensor which detects a sync signal serving as a write start reference of the laser beam in the main scanning direction corresponding to the longitudinal direction of the photosensitive member.
However, the image forming apparatus described above has the following problem when the apparatus does not measure the light amount of a back light, but measures the light amount of a laser beam directed from the light source to the photosensitive member and performs a light amount control of the laser beam.
More specifically, in the arrangement in which a part of a light amount obtained by splitting the laser beam emitted from the VCSEL using the half mirror is detected, the light amount of the laser beam used for scanning a photosensitive member is reduced to half due to the splitting of the laser beam. Since the VCSEL structurally has a weak point that the laser resonator length is short and the maximum output is small, a light amount large enough to scan the photosensitive member may not be obtained.
In this arrangement, in order to prevent the remaining part of split laser beams from scanning on the photosensitive member during light emission for APC, it is necessary to perform APC within a non-scanning area in which no laser beam scans on the photosensitive member. This is because when a latent image may be formed on the photosensitive member by light emission for APC, toner is placed at an unintended position other than the position where an image should be formed at the time of development. This may cause smearing on an end portion of an output sheet, cleaning failure on the photosensitive member, and the like.
In the arrangement in which the light amount control is performed using the BD sensor, since light amount detection is performed using scanning light reflected by the polygon mirror, the time during which a laser beam irradiates on the BD sensor is very short, and then there is a problem that a light amount sufficient for accurate light amount detection may not be obtained.